Because of its clean burning qualities and convenience, natural gas has become widely used in recent years. Many sources of natural gas are located in remote areas, great distances from any commercial markets for the gas. Sometimes a pipeline is available for transporting produced natural gas to a commercial market. When pipeline transportation is not feasible, produced natural gas is often processed into liquefied natural gas (which is called “LNG”) for transport to market.
In the design of an LNG plant, one of the most important considerations is the process for converting the natural gas feed stream into LNG. Currently, the most common liquefaction processes use some form of refrigeration system. Although many refrigeration cycles have been used to liquefy natural gas, the three types most commonly used in LNG plants today are: (1) the “cascade cycle,” which uses multiple single component refrigerants in heat exchangers arranged progressively to reduce the temperature of the gas to a liquefaction temperature; (2) the “multi-component refrigeration cycle,” which uses a multi-component refrigerant in specially designed exchangers; and (3) the “expander cycle,” which expands gas from feed gas pressure to a low pressure with a corresponding reduction in temperature. Most natural gas liquefaction cycles use variations or combinations of these three basic types.
The refrigerants used may be a mixture of components such as methane, ethane, propane, butane, and nitrogen in multi-component refrigeration cycles. The refrigerants may also be pure substances such as propane, ethylene, or nitrogen in “cascade cycles.” Substantial volumes of these refrigerants with close control of composition are required. Further, such refrigerants may have to be imported and stored imposing logistics requirements. Alternatively, some of the components of the refrigerant may be prepared, typically by a distillation process integrated with the liquefaction process.
The use of gas expanders to provide the feed gas cooling thereby eliminating or reducing the logistical problems of refrigerant handling has been of interest to process engineers. The expander system operates on the principle that the feed gas can be allowed to expand through an expansion turbine, thereby performing work and reducing the temperature of the gas. The low temperature gas is then heat exchanged with the feed gas to provide the refrigeration needed. Supplemental cooling is typically needed to fully liquefy the feed gas and this may be provided by additional refrigerant systems, such as secondary cooling loops. The power obtained from cooling expansions in gas expanders can be used to supply part of the main compression power used in the refrigeration cycle. Though a typical expander cycle for making LNG can operate at the feed gas pressure, typically under about 5,516 kPa (800 psia), a high pressure primary cooling loop had been found to be particularly promising. See, for example, WO 2007/021351. It has also been discovered that adding external cooling to such a primary cooling loop provides additional advantages in many situations. See PCT/US08/02861.
Because expander cycles result in a high recycle gas stream flow rate and resulting high cooling load, introducing inefficiencies for the primary cooling (warm) stage, gas expander processes such as described above further cool the feed gas after it has been pre-cooled using a refrigerant in a secondary cooling unit. For example, U.S. Pat. No. 6,412,302 and U.S. Pat. No. 5,916,260 present expander cycles which describe the use of nitrogen as refrigerant in the sub-cooling loop. The primary (warm-end) expander cooling loop operates at low pressure and therefore limits the fraction of the feed gas cooling load provided by this primary loop. Consequently, a nitrogen (or nitrogen-rich) refrigerant is required in the sub-cooling loop. WO 2007/021351 (above) uses a portion of the flash gas derived from the feed gas in the final separation unit. Thus, generally, an element in expander cycle processes is the requirement for at least one second refrigeration cycle to sub-cool the feed gas before it enters the final expander for conversion of much, if not all, remaining gaseous feed to LNG.
Though this process performs comparably to alternative mixed external refrigerant LNG Production processes, including mixed expander-refrigerant processes, it has been of interest to improve the efficiency of the process of expander cycles for making LNG. In particular it has been of interest to use less fuel and reduce the power generation equipment required, especially for hard to reach locations, such as offshore or in environmentally severe onshore locations.
Other potentially relevant information may be found in International Publication No. WO2007/021351; Foglietta, J. H., et al., “Consider Dual Independent Expander Refrigeration for LNG Production New Methodology May Enable Reducing Cost to Produce Stranded Gas,” Hydrocarbon Processing, Gulf Publishing Co., vol. 83, no. 1, pp. 39-44 (January 2004); U.S. App. No. US2003/089125; U.S. Pat. No. 6,412,302; U.S. Pat. No. 3,162,519; U.S. Pat. No. 3,323,315; and German Pat. No. DE19517116.